1. Field of the Invention
The present invention relates to a voltage regulator including an overheat protection circuit.
2. Description of the Related Art
In general, a voltage regulator is configured to supply a current depending on a load of an electronic device connected to an output port, and energy consumption due to heat generation leads to electric power loss. Further, when a load current increases, a temperature of the voltage regulator rises excessively and the voltage regulator itself may be damaged. Accordingly, there is provided an overheat protection circuits configured to prevent the voltage regulator from reaching a predetermined temperature or higher.
Here, a related-art voltage regulator including an overheat protection circuit is described (e.g., see Japanese Patent Application Laid-open No. 2005-100295).
FIG. 3 is a circuit diagram of a related-art voltage regulator 200.
The voltage regulator 200 includes an overheat protection circuit 123 including a temperature sensing circuit 115, a reference voltage circuit 114, a comparator circuit 103, and transistors 104 and 110, and has the following configuration.
The temperature sensing circuit 115 is formed of a constant current circuit 101 and a diode 102, and is configured to output a voltage VF from a node between the constant current circuit 101 and the diode 102.
The reference voltage circuit 114 is formed of a reference voltage 106, a voltage follower circuit 105, and bleeder resistors 107, 108, and 109, and is configured to output a voltage VREF from a node between the resistors 107 and 108.
The comparator circuit 103 is configured to compare the voltage VF, which is output of the temperature sensing circuit 115, and the voltage VREF, which is output of the reference voltage circuit 114, to each other, and to output the result of the comparison. Output of the comparator circuit 103 is input to a gate of the transistor 104 and a gate of the transistor 110.
The transistor 104 has a source connected to a power supply terminal, and a drain connected to a gate of an output transistor (output driver) 111 of the voltage regulator 200. The transistor 110 has a source connected to a ground terminal, and a drain connected to a node between the resistors 108 and 109.
A voltage divider circuit formed of the resistors 112 and 113 is connected between a drain of the output transistor 111 and the ground terminal.
An error amplifier circuit 116 is configured to receive a divided voltage from the voltage divider circuit and a voltage of a reference voltage 117, and has an output terminal connected to the gate of the output transistor 111.
Temperature characteristics of the temperature sensing circuit 115 are based on temperature characteristics of a forward voltage of the diode 102, and the output voltage VF has a characteristic of being substantially −2 mV/° C. The output voltage VREF of the reference voltage circuit 114 may be set to any voltage value through adjustment of the bleeder resistors 107, 108, and 109 by trimming.
In a normal state in which an overheated state is not detected, VF>VREF holds. Thus, the output of the comparator circuit 103 becomes a HIGH state, and the transistor 104 is turned off. As a result, a gate voltage of the output transistor 111 reaches a voltage of the output terminal of the error amplifier circuit 116. Consequently, the output transistor is turned on, and an output voltage VOUT having a predetermined potential is output.
When the overheated state is detected, on the other hand, VREF>VF holds. Thus, the output of the comparator 103 becomes LOW, and the transistor 104 is turned on. As a result, the gate voltage of the output transistor 111 reaches a power source voltage, and the output transistor 111 is thus turned off Consequently, the output voltage VOUT has a ground potential.
As described above, when the overheat protection circuit 123 does not detect the overheated state, the related-art voltage regulator 200 normally operates and outputs, from the output transistor 111, the predetermined voltage VOUT having a power source potential or lower. When the overheat protection circuit 123 detects the overheated state, the related-art voltage regulator 200 turns off the output transistor 111 so that the output voltage VOUT has the ground potential. In this way, the voltage regulator itself may be protected from an excessive temperature rise thereof.
The transistor 110 is provided in order that the voltage regulator is changed from the overheated state to the normal state, and reversely from the normal state to the overheated state at different temperatures, that is, the voltage regulator has hysteresis.
In high-voltage and high-current voltage regulators, a large amount of electric power is lost due to a translational increase in load current under a high voltage state. The electric power is lost mainly due to energy consumption by an output driver generating heat. However, when the output driver and a diode of a temperature sensing circuit are arranged on a chip at positons away from each other, there is a temperature difference between a temperature near the center of the output driver at which heat is generated by the largest amount and a temperature at the diode of the temperature sensing circuit, due to thermal gradient.
In the related-art voltage regulator 200 of FIG. 3, there is a fear in that at a time when the overheat protection circuit 123 detects a predetermined overheated state, a temperature near the center of the output driver (output transistor 111) at which heat is generated by the largest amount may reach a temperature of the predetermined overheated state or higher, and exceed a heat resistant temperature of the output driver 111 to damage the output driver 111.